My neighbour is building an extension about 10cm away from my external wall on one side. It runs parallel to my house for about 10 metres. So there should have been a gap running between the two houses about this wide after their extension is built. They approached me before and said they wanted to brick up both sides of this gap, they couldn't say what they were going to put onto the top of it to cover the top of this gap so I was dubious. Short version I said no because a) I had previous issues with them disputing the physical boundary wall running between our two gardens which on the deeds are marked with a 'T' on my side and b) on their plans they submitted to the Council they showed intent to build behind the boundary line and also c) they were very aggressive about the boundary wall issue which was ongoing for weeks and weeks causing no end of stress and sleepless nights so I just wanted to stay away from them in every way possible.

Anyway the above is a different matter. Even though I had said no, I have noticed they have now started bricking up the gap between our two properties. The brickwork is so badly done it clearly looks out of line with my front wall and looks like part of MY front wall brickwork which has been badly done because my house sits a bit behind theirs. They have also bricked up the back of the gap. The brickwork and mortar is actually stuck to my wall all up these gaps (front and back). They have also poured concrete into the 10 metre long gap to a height of about 30cm which has obviously dried onto my wall and I am worried it was gone over my damp course line. They have yet to reach full height with the brickwork and I don't know what they will fill the rest of it with. Bearing in mind this external wall they've attached their brickwork onto is part of my house and was previously completely separated from them, are they allowed to do this?

They have now also completely gone beyond the line of the physical boundary wall running behind our gardens by a full 4 or 5 inches as my build does not go up to the boundary line at the back part so that brickwork they did there is technically on my land. (They were disputing ownership of that wall which still hasn't been resolved previously but that's a different matter).

What can I do to stop them bricking this up further or to get them to remove it? Will going the legal route aggravate the situation? My legal adviser told me last week that that they have already committed criminal damage by knocking down part of the boundary wall for their build as on my deeds it is marked as my responsibility and has been there for 40 years. I have to phone the legal team tomorrow about this issue but not sure if they're about at the weekend. They had previously advised me to talk to these neighbours first but it seems these people can't or don't want to understand 'boundaries' of any kind.